With the rapid aging of the society, diseases associated with tissue degeneration and tissue injury are rapidly increasing. Said diseases include cerebral infarction, myocardial infarction and renal failure that develop in an age-dependent manner due to the metabolic syndrome, Alzheimer's disease, Parkinson's disease and osteoporosis that are induced by age-related internal changes of the tissue, and the like. Type I diabetes, multiple sclerosis and rheumatoid arthritis induced by autoimmune diseases as well as burns and spinal injuries induced by wounds are also diseases characterized by tissue degeneration and tissue injury. As methods of treating such diseases resulting from tissue degeneration and injury, various regenerative medical techniques are being developed now.
Regenerative medicine is roughly divided into two methods: the induced regeneration method in which endogenous stem cells in patients are activated with a drug etc., and the cell replacement therapy in which stem cells or somatic cells induced from stem cells or tissues are transplanted. Specifically, in diseases accompanied by chronic inflammation and diseases in elderly individuals, the induced regeneration method does not work well due to reduced function of stem cells from the patient per se, and thus the development of the cell replacement therapy is imperative. In order to treat diseases resulting from tissue degeneration and injury by a cell replacement therapy, a large amount of stem cells or somatic cells induced from stem cells generally need to be prepared as materials for transplantation. For this purpose, stem cells that can differentiate into various tissues and that can self-replicate for a long time are indispensable for the development of a cell replacement therapy.
As stem cells that satisfy these conditions, there have been reported ES cells or EG cells that can be induced from fertilized eggs or primordial germ cells. However, in order to perform the cell replacement therapy safely and efficiently, it is necessary to prepare ES cells or EG cells comprising the genome of the patient per se that can circumvent the immunological rejection of transplanted cells.
As a method of preparing ES cells comprising the genome of the patient per se, a method of nuclear transplantation in which the nucleus of an egg is replaced with that of a somatic cell of the recipient has been investigated in animals such as mice. However, the success rate of nuclear transplantation is still low, and no success has been made in humans. Separately, there is a report on establishment of the iPS (induced pluripotent stem) cells having a property close to that of ES cells by introducing four genes of Oct3/4, Sox2, Klf4 and c-Myc into fibroblasts derived from mouse skin (Cell 126: 1-14, Aug. 25, 2006). However, the rate of iPS induction is low, and it has not been successful in humans.